sshd_config: clarify password authentication options

Passwords may be accepted by both the PasswordAuthentication and
KbdInteractiveAuthentication authentication schemes. Add a reference to
the latter in the description/comment for PasswordAuthentication, as it
otherwise may seem that "PasswordAuthentication no" implies passwords
will be disallowed.

This situation should be clarified with more extensive documentation on
the authentication schemes and configuration options, but that should be
done in coordination with upstream OpenSSH. This is a minimal change
that will hopefully clarify the situation without requiring an extensive
local patch set.

PR: 263045
Reviewed by: manu (earlier version)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35272

(cherry picked from commit 9f009e066f088e2c31442db31d2a85001040abfe)